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Best Answer: For average speed over a long distance it takes the distance traveled and divides it by the time taken - exactly what the other answers said.

For instantaneous speed, how fast you were going at one point in time, this approach doesn't work. Which is a pity because that's exactly what you want to know if you want to check your drivers aren't speeding.
Each GPS position has an error of up to 5 meters, that means that if you take two positions a second apart take the distance between them and calculate the speed you could be off by up to 10 meters per second or about 22.5 mph.

The way a GPS actually calculates speed at any given moment is by measuring the doppler shift in the signals from the satellites. Just as a car engine or police siren sounds higher pitch when it's heading towards you than when it's heading away the signals from the satellites change very slightly depending on how fast you are moving towards or away from them.
If you know where you are (you have a GPS position) and you know where and how fast the satellites are moving (they broadcast that information) then by measuring the doppler on the signals you can calculate how fast the receiver is moving far more accurately than by taking the difference between two positions.

How accurate is it? As with anything to do with GPS it depends on how many satellites are being tracked and how strong the signals are. It would be rare for it to be more than one or two mph wrong.

Basically it logs a number of lat/long points, and the GPS (atomic based) time it was at those points, and does the maths to calculate the distance between points, and the time it took to travel between those points. From there the rate can easily be calculated.

I think it is reasonably accurate. However, I think a proper spedometer is more accurate, provided the original sized tires are used, or the computer programmed to the actual tire size and all that.

It would be distance divided by time and since a gps is sort of like a computer probably pretty accurate

Anonymous · 8 years ago

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· just now

It just figures it by how fast your position changes. It's extremely accurate as long as it has a few seconds of data. It's only weakness is measuring speed over very short stretches, like a fraction of a second and measuring speeds under 3mph.